The invention relates to a control device for stabilizing the rolling motion of a vehicle, having control elements arranged between the wheel carriers or wheel suspension members and the vehicle body which generate a stabilizer torque at the front axle and a stabilizer torque at the rear axle as a function of at least one cross-dynamic motional quantity.
A control device of this type is known from German Published Unexamined Patent Application (DE-OS) 28 44 413. In this known control device, one stabilizer torque respectively is generated at the front axle and at the rear axle of the vehicle by means of hydraulically adjustable spring struts. These stabilizer torques at the front axle and at the rear axle of the vehicle counteract the rolling motion of the vehicle body which occurs during cornering. The adjustment of the hydraulic spring struts takes place by means of electrohydraulic valves which indicate the inflow or the outflow of the hydraulic fluid to or from the hydraulic spring struts corresponding to their respective degree of openness The control inputs of the electrohydraulic valves are electrically connected with the output of a controller which generates the control signals for the electrohydraulic valves as a function of the measured lateral acceleration of the vehicle
It is a disadvantage of this known control device that the hydraulically adjustable spring struts of one vehicle side are acted upon by the same hydraulic pressure at the front axle and at the rear axle. As a result, during a fast steering of the vehicle into a cornering situation, dynamic wheel load fluctuations will occur which may lead to fluctuations of the cornering forces at the front axle and at the rear axle of the vehicle. These, in turn, lead to pendulum motions of the vehicle around the vertical axis of the vehicle and thus to an unstable handling of the vehicle
It is therefore an object of the invention to provide a control device of the initially mentioned type by means of which, in addition to a minimizing of the roll angle of the vehicle body, an optimal handling can be achieved of a vehicle equipped with the control device according to the invention.
According to the invention, this object is achieved by an arrangement wherein the stabilizer torques M.sub.V and M.sub.H at the front axle and at the rear axle are generated as a function of measured or determined quantities of the yaw velocity .psi.(t), of the roll angle, and of the roll angle rate .gamma.(t) according to the following principles: EQU M.sub.V (t)=k.sub.11 (V(t)i.sub.m).multidot..psi.(t)+k.sub.12 .multidot..gamma.(t)+k.sub.13 .multidot..gamma.(t) (I) EQU M.sub.H (t)=k.sub.21 (V(t)i.sub.M).multidot..psi.(t)+k.sub.22 .multidot..gamma.(t)+k.sub.23 .multidot..gamma.(t) (II)
wherein k.sub.11 and k.sub.21 are coefficients depending on the driving . speed V (t) and on the stabilizer torque distribution ##EQU1## during steady-state cornering, and k.sub.12, k.sub.22, k.sub.13 as well as k.sub.23 are vehicle-specific constant coefficients which are larger than zero.
As input quantities for the generating of the stabilizer torques, the control device according to the invention requires the driving speed .psi. as well as the variable quantities of yaw velocity, roll angle .gamma. and roll angle rate .gamma.. These variable quantities, corresponding to their weighting by the coefficients k.sub.ij with i=1, 2 and j=1, 2, 3, determine the stabilizer torque which, in each case, effects the front axle and the rear axle. The coefficients k.sub.12, k.sub.22, k.sub.13 and k.sub.23 of the rolling motion, are constant and thus independent of the driving speed .gamma. and of the steady-state stabilizer torque distribution i.sub.M between the front axle and the rear axle. By means of these coefficients, the dynamic behavior and the steady-state value of the roll angle .gamma. can be indicated.
The dynamic behavior of the yaw velocity .psi., of the sideslip angle .beta. and of the resulting quantities, such as the lateral acceleration, can be indicated by means of the coefficients k.sub.11 (V (t), i.sub.M) and k.sub.21 (V (t), i.sub.M). The values or the characteristic-curve fields of the coefficients may be determined easily by means of simulation studies. The characteristic-curve fields of coefficients k.sub.11 and k.sub.21, for a certain vehicle type, are shown as an example in FIGS. 1 and 2. FIG. 1 shows that the coefficient k.sub.11 at a constant steady-state stabilizer torque distribution ##EQU2## rises almost linearly with an increasing driving speed V. FIG. 1 also shows that the characteristic k.sub.11 lines, with an increasing value of the steady-state stabilizer torque distribution i.sub.M, are shifted in the direction of lower k.sub.11 values. FIG. 2 shows that coefficient k.sub.21, at a constant steady-state stabilizer torque distribution i.sub.M falls off almost linearly with an increasing driving speed FIG. 2 also shows that the characteristic k.sub.21 lines, with an increasing steady-state stabilizer torque distribution i.sub.M, are shifted in the direction of higher k.sub.21 values.
By means of a determination of the individual coefficients which is dependent on the respective vehicle type, particularly of coefficients k.sub.11 and k.sub.21, the damping behavior of the essential cross-dynamic motion quantities, such as the yaw velocity .psi., the sideslip angle, the roll angle and the lateral acceleration of the "controlled" vehicle can clearly be improved in comparison to a vehicle with a conventional stabilizing of the rolling motion. This improvement increases with an increasing lateral acceleration. The dynamic behavior of the vehicle in the low lateral acceleration range - the driver is normally familiar with this range--is maintained also in the upper lateral acceleration range. The driver can therefore control the "controlled" vehicle more easily. These improvements are caused by the stabilizer torques M.sub.V and M.sub.H because of the fact that they distribute the changes of the normal wheel force occurring during cornering in such a manner between the front axle and the rear axle of the vehicle that, irrespective of the driving condition, an optimal distribution of the cornering forces is always ensured between the front axle and the rear axle. By means of the control device according to the invention, therefore, in addition to a minimizing of the roll angle during cornering, an optimal handling and roll steer effect is achieved of a vehicle equipped with the control device according to the invention.
The steady-state stabilizer torque distribution ##EQU3## which occurs during steady-state cornering, is preferably selected to be not constant, but is determined as a function of the longitudinal acceleration of the vehicle V (t) and of the vehicle capacity weight .DELTA. m. As a result, it is achieved that the stabilizer torques M.sub.V and M.sub.H and irrespective of the acceleration and of the capacity weight of the vehicle, are always distributed between the front axle and the rear axle in such a manner that the achievable lateral acceleration is maximized and at the same time a safe driving condition is maintained Thus, for example, a higher stabilizer torque M.sub.V may be applied at the front axle of the vehicle during braking than during the acceleration of the vehicle. As a result, the vehicle tends to corner less. In addition, for example, in the case of a vehicle with a high capacity weight in the rear area, a higher stabilizer torque can be applied at the rear axle of the vehicle than in the case of an unloaded vehicle.
Simulation studies for various vehicle types have shown that it is advantageous for coefficient k.sub.12 to be larger than coefficient k.sub.13, and for coefficient k.sub.22 to be larger than coefficient k.sub.23.
It was also found to be advantageous for the respective ratio of coefficients k.sub.12 to k.sub.22 and k.sub.13 to k.sub.23, in the case of standard dimensions, to be in the range of between 0.2 and 0.8.
In a further development of the invention, the body roll angle .gamma..sub.A (t) of the vehicle body relative to the wheel carriers or wheel suspension members and/or its derivative with respect to time .gamma..sub.A (t) is used for generating the stabilizer torques M.sub.V and M.sub.H at the front axle and the rear axle instead of the inertial roll angle .gamma. (t) and/or the inertia roll angle rate .gamma. (t). This advantage of this further development of the invention consists of the easier measurability of the body roll angle .gamma..sub.A (t) in comparison to the inertial roll angle .gamma. (t). The body roll angle .gamma..sub.A (t) may be determined with very low measuring expenditures, for example, from the bump travel of the vehicle body relative to the wheel carriers or the wheel suspension members. This is also true for the body roll angle rate .gamma..sub.A (t).
In order to keep the number of signals to be measured as low as possible, according to a further development of the invention, the roll angle rate .gamma. (t) is obtained by status filtering or, when a digital computer is used, by the numerical differentiation of the roll angle .gamma. (t).
The determination of the longitudinal acceleration V (t) of the vehicle by status filtering or, when a digital computer is used, by the numerical differentiation of the driving speed V (t) serves the same purpose.
According to a further development of the invention, the stabilizer torques M.sub.V and M.sub.H at the front axle and at the rear axle of the vehicle are generated also as a function of the sideslip angle .beta. (t) and/or of the steer angle .delta. (t), in addition to the above-mentioned input quantities. This further development of the invention has excellent control dynamics.
If, according to a further development of the invention, individual cross-dynamic motional quantities, particularly the roll angle .gamma., the yaw velocity .psi., or the roll angle rate .gamma., are determined by means of a vehicle-specific model of cross-dynamics (simulator) as a function of the steering angle .delta. (t), these cross-dynamic motional quantities no longer have to be measured If all cross-dynamic motional quantities of principles (I) and (II) of the stabilizer torques at the front axle and the rear axle are generated by means of the vehicle-specific model of cross-dynamics as a function of the steering angle, and these estimated values are used instead of the real motional quantities in principles (I) and (II) for the stabilizer torques, a controlling device is obtained on the basis of the control device, by means of which, as a function of the steering angle, of the driving speed and possibly of the vehicle capacity weight, the stabilizer torques are generated at the front axle and the rear axle.
This type of a vehicle-specific model of cross-dynamics (simulator) is determined by the following mathematic equations. EQU .beta.(t)=a.sub.11 (V).multidot..beta.(t)+a.sub.12 (V).multidot..psi.(t)+a.sub.13 (V).multidot..gamma.(t)+a.sub.14 (V).multidot..gamma.(t)+b.sub.1 (V).multidot..delta.(t) EQU .psi.(t)=a.sub.21 .multidot..beta.(t)+a.sub.22 (V).multidot..psi.(t)+a.sub.23 .multidot..gamma.(t)+a.sub.24 .multidot..gamma.(t)+b.sub.2 .multidot..delta.(t) EQU .gamma.(t)=a.sub.31 .multidot..beta.(t)+a.sub.32 (V).multidot..psi.(t)+a.sub.33.multidot..gamma. (t) +a.sub.34 .gamma.(t)+b.sub.3 .multidot..delta.(t)
wherein the coefficients a.sub.ij, j=1, and a.sub.i2, i=1, . . . , 3 depend on the driving speed V(t), all other coefficients are constant, .beta.(t) is the derivative of the sideslip angle .beta.(t), .psi.(t) is the yaw acceleration, i.e. the second derivative of the yaw .psi.(t), and .gamma.(t) is the roll angle acceleration, i.e. the second derivative of the roll angle .gamma.(t). When a digital computer is used instead of an analog computer, this analog vehicle-specific model of cross-dynamics may naturally be replaced by its equivalent model with discrete values in time.
Principles (I) and (II) for the generating of stabilizer torques at the front axle and at the rear axle naturally apply to an implementation of a controller which is constant in time as well as for a controller which has discrete values in time on a digital computer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.